ORIGINAL PAPER

Effect of Ni/Mn ratio on the performance of LiNixMn2−xO4cathode material for lithium-ion battery

Wan Ren & Rui Luo & Zu-shan Liu & Xi-you Tan &

Zhi-yong Fu & Shi-jun Liao

Received: 12 February 2014 /Revised: 10 March 2014 /Accepted: 15 March 2014# Springer-Verlag Berlin Heidelberg 2014

Abstract Lithium nickel manganate is recognized as a typeof promising cathode material for lithium-ion battery, due toits advantages such as high voltage, high power density, andrelative lower cost. In this paper, a series of LiNixMn2−xO4

cathode materials with various molar ratio of Ni/Mn havebeen prepared with a co-precipitation method, followed by asolid state reaction, and the effect of the molar ratio of Ni/Mnon the structure and properties of materials are intensivelyinvestigated by means of X-ray diffraction (XRD), Fouriertransform infrared spectrometer (FTIR), scanning electronmicroscopy (SEM), and performance measurements, etc. Itis revealed that all the samples with x from 0 to 0.5 have well-defined spinel structure and fit well to Fd-3 m space group.With the increase of the molar ratio of Ni/Mn, the diffractionpeaks shift to higher angle slightly and the lattice parameterdecreases gradually by the XRD results. Furthermore, it isfound that the capacity at the 4.0 V plateau decreases while thecapacity at 4.7 V plateau increases with the increase of theratio of Ni/Mn, and the total discharge capacity shows growthtrend with the increase of Ni content. It is important that all thesamples with various molar ratio of Ni/Mn exhibit good cyclicstability. Based on the experimental results, we suggest thatthe Ni may incorporate into the lattice of LiMn2O4 substitut-ing ofMn. The plateau at 4.7 V is related to the Ni ions and theplateau at 4.0 V is related to the Mn ions in the materials.

Keywords Ni/Mn ratio . Lithium nickelmanganate .

Cathode . Lithium-ion battery

Introduction

Lithium nickel manganate (LiNi0.5Mn1.5O4) is recognized as apromising cathode material for lithium-ion battery for powerand energy storage applications, because of its high voltage,high power density, and low cost [1]. Various methods such assolid state reaction [2], spray drying process [3], co-precipitation [4], and sol-gel [5] have been developed for thepreparation of LiNi0.5Mn1.5O4 materials. The spinel structureof LiNi0.5Mn1.5O4 offers 3-dimensional lithium-ion diffusion,allowing fast insertion/removal of lithium ions duringdischarge/charge processes [6]. Depending on the arrange-ment of Mn4+ and Ni2+ ions in the lattice, LiNi0.5Mn1.5O4 isknown to have two forms: disordered spinel with a Fd-3 mspace group and 1:3 ordered spinel with a P4332 space group[7]. Ni2+/4+ redox couple appears in this cathode material,which is active in the high voltage range around 4.7 V.However, the high voltage of this material brings the chal-lenge for the stability of electrolyte, and the aggressive sidereactions may degrade the active material, leading to irrevers-ible capacity loss of the cathode material. Thus, many re-searchers attempted to improve the performance of this mate-rial through modifications, such as doping ions [8, 9], coatingother materials [10-12], and engineering its particle size [13,14], etc. Due to Ni plays a crucial role for the LiNixMn2−xO4

cathode materials, it is meaningful to reveal and understandthe role of Ni for the improvement of the material.

Regarding the role of Ni or the effect of Ni content onthe structure and performance of LiNixMn2−xO4 cathodematerial, a few literatures can be searched to address thisissue [15-18]. For example, Zhong et al [15], synthesizedLiNixMn2−xO4 by sol-gel and solid-state method, respec-tively. They found that the oxidation state of Ni in thesesamples was +2, and these materials could be written asLi+1Ni+2xMn+31−2xMn+41+xO

−24. Wei et al [17] found that

Ni cations moved to the 8a sites in heavily substituted

W. Ren : R. Luo : Z.<s. Liu :X.<y. Tan : Z.<y. Fu : S.<j. Liao (*)The Key Laboratory of Fuel Cell Technology of GuangdongProvince and The Key Laboratory of New Energy Technology ofGuangdong Universities, School of Chemistry and ChemicalEngineering South China University of Technology,Guangzhou 510641, Chinae-mail: chsjliao@scut.edu.cn

IonicsDOI 10.1007/s11581-014-1114-3

LiNixMn2−xO4 (x≥0.3) and Ni2+ were partially oxidized toNi3+. Clearly, current researches for LiNixMn2−xO4 were justconfined in the case of x≤0.2 [18, 19], more work is needed toelucidate the effect of the ratio of Ni/Mn on the structure andperformance intensively and systematically.

For the purpose of exploring the role of Ni in LiNixMn2−xO4

cathodematerial, we investigated the effect of Ni content on thestructure and performance of the material. In this work, weprepared a series of spinel lithium nickel manganate materialswith various molar ratio of Ni/Mn by a two-stage approach, inwhich the Ni and Mn binary oxide was prepared by a co-precipitation method, followed by calcining the mixture of thebinary oxides and lithium carbonate. With various Ni/Mn ra-tios, the effect of the ratio of Ni/Mn on the structure andperformance of the lithium nickel manganate material has beeninvestigated systematically. It is found that the ratio of Ni/Mnonly affect the structure of the materials slightly, but it affect theperformance (including the capacity and the capacity distribu-tion at various plateaus) of the material significantly.

Experimental

Preparation

Manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O,99 . 0 %, Ke rme l ) , n i cke l a ce t a t e t e t r ahyd r a t e(Ni(CH3COO)2 ·4H2O, 98.0 %, Kermel), and lithiumcarbonate(Li2CO3, 97.0 %, Kermel) were used as raw mate-rials for the preparation of lithium nickel manganate, andoxalic acid dihydrate (C2H2O4·2H2O, 99.5 %, Enox) wasused as precipitant.

LiNixMn2−xO4 (0≤x≤0.5) materials were synthesized by atwo-stage approach reported previously [20]. In the first step,a nickel and manganate binary oxalate co-precipitate wasprepared by adding oxalic acid solution to the solution con-taining manganese acetate tetrahydrate and nickel acetatetetrahydrate, stirring and aging at room temperature for 6 h,followed by filtrating, washing, and drying overnight. Aftercalcining the oxalate at 550 °C for 5 h, spinel-structuredmanganese-nickel (Mn-Ni) binary oxide was obtained. Inthe second step, LiNixMn2−xO4 (0≤x≤0.5) materials wereprepared by mixing and ball milling the mixture of the binaryoxide and lithium carbonate, followed by calcining at 900 °Cin air for 15 h with a heating rate of 5 K/min.

Materials characterization

The materials were identified byX-ray diffraction (XRD) witha D8 Advance X-ray powder diffraction meter (Bruker,Germany) using Cu-Kα radiation. The particle morphologieswere observed using scanning electron microscopy (SEM,JSM-6380, JEOL/EO, Japan). The Fourier transform infrared

(FTIR) analysis was performed at a Fourier transform infraredspectrometer (Tensor 27, Bruker, Germany); the samples wereprepared by diluting a small amount of powders in 100 mg ofKBr powders.

Electrochemical testing

Electrochemical evaluation of materials was performed withCR2016 coin cells using Li metal as the negative electrode.The positive electrode was prepared as follows: firstly, theprepared materials (78 wt%), acetylene black (12 wt%), andpolyvinylidene fluoride (PVDF, 10 wt%) binder were mixedin N-methyl-2-pyrrolidone (NMP) to form a slurry. Then theslurry was coated onto an aluminum foil, which served as acurrent collector, followed by vacuum drying at 120 °C for12 h in a vacuum oven. Finally, the cathode material-coatedfoils were pressed under 15 M Pa for 3 min, with a finalmaterial loading about 7–10 mg cm−2. The testing cells wereassembled with a lithium plate as the negative electrode, and1 M LiPF6 solution, in which a mixture of ethylene carbonate(EC) and dimethyl carbonate (DMC) (1:1 in volume) wasused as the solvent—as the electrolyte. The cells were testedby galvanostatic charge–discharge cycling between 3.5 and4.9 V (vs. Li+/Li) on a battery testing system (Neware, CT-3008, China) with various charge–discharge rates at roomtemperature.

Results and discussion

Structure characterization

The XRD patterns of the LiNixMn2−xO4 (0≤x≤0.5) samplesare displayed in Fig. 1a. Almost no impurities like nickeloxides and manganese oxides are observed in the products,and all the samples exhibit a high degree of crystallinity,implying that this two step method [20] is a useful way forthe synthesis of LiNixMn2−xO4 (0≤x≤0.5) cathode materialsfor lithium-ion battery. Comparing the XRD patterns ofLiMn2O4 and LiNixMn2−xO4, we can see that no significantdifference can be found between their base diffraction pat-terns, but the lattice parameters were changed slightly, indi-cating that the Ni may enter the lattice of LiMn2O4 to form theLiNixMn2−xO4.

From Fig. 1b, we can see that the diffraction peaks shift tothe higher angle slightly with the increase of the ratio of Ni/Mn,revealing that the microstructure of materials is slightlychanged with the addition of nickel. Table 1 shows theaverage lattice parameters calculated from the (111), (311)and (400) peaks of each sample with various of Ni/Mn ratios,clearly, with the increase of the ratio of Ni/Mn, thelattice parameter decreases gradually (Fig. 1c), which can beexplained by the ionic size effect [21]. This should be attributed

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to the ionic radius difference amongMn3+(0.79Å),Mn4+(0.67Å),andNi2+(0.83Å );Mn4+ andNi2+ ions take the place of twoMn3+

ions in this substitution scheme and the average ionic radius ofNi2+ and Mn4+ is smaller than the ionic radius of Mn3+ [22, 23] .

So, this lattice contraction is also an indication of the successfulsubstitution of Ni for Mn in the lattice. And it is recognized to bebeneficial to enhance the cycling behavior of materials [18, 24].

Two types of phases have been reported for LiNixMn2−xO4

(0≤x≤0.5), which is P4332 space group and Fd-3 m spacegroup. The former represents the Mn and Ni ions ordering inthe 12d and 4b sites, while the latter indicates the randomoccupation of Ni and Mn ions in the 16d site of octahedral[25, 26]. It is difficult to identify these two groups by usingX-ray diffraction, but FTIR is an efficient technique todetermine the cation ordering. Figure 2 shows the FTIRspectra of LiNixMn2−xO4 with various Ni content (x=0,0.1, 0.2, 0.3, 0.4, 0.5, respectively). It was reported thatordering of Mn4+ and Ni2+ ions leads to a higher intensityband at 588 cm−1 and a lower one at 624 cm−1, and theabsorption peak at 470 cm−1 is sharp in the P4332 spacegroup for the spinel LiMn2O4 material [16, 27-29]. So, allof our synthesized samples fit well to Fd-3 m space group.With the increase of the ratio of Ni/Mn, the absorptionintensity at 588 cm−1, which can be assigned to the Ni-Oband, increases gradually. While, the absorption peaks at515 and 613 cm−1, which can be assigned to Mn-O band,shift to 505 cm−1 and 624 cm−1, respectively, when the Nicontent change from zero to x=0.5, indicating the substitu-tion of Mn with Ni affected the Mn-O band significantly.

SEM of material

Figure 3 displays the SEM images of the LiNixMn2−xO4

samples with x of 0, 0.2, and 0.4, respectively, three samplesare all well crystallized with a spinel structure, and the parti-cles size is about 1.5 μm with uniform particle size distribu-tion. No obvious differences can be observed for these threesamples, indicating that the morphology of particles is notaffected remarkably with the change of the Ni content.

Performances of materials

Figure 4a shows the discharge curves of LiNixMn2−xO4 sam-ples (0≤x≤0.5). For LiNi0.5Mn1.5O4 sample, it presents a

Table 1 Lattice parameters of LiNixMn2−xO4 (0≤x≤0.5) cathodematerials

Sample Latticeparameter (Å)

Sample Latticeparameter (Å)

LiMn2O4 8.2078 LiNi0.30Mn1.70O4 8.1824

LiNi0.05Mn1.95O4 8.2138 LiNi0.35Mn1.65O4 8.1717

LiNi0.10Mn1.90O4 8.1974 LiNi0.40Mn1.60O4 8.1666

LiNi0.15Mn1.85O4 8.1958 LiNi0.45Mn1.55O4 8.1664

LiNi0.20Mn1.80O4 8.1849 LiNi0.50Mn1.50O4 8.166

LiNi0.25Mn1.75O4 8.1831

10 20 30 40 50 60 70

Inte

nsi

ty (

a.u

.)

2 Theta

x=0.00

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(531

)

(440

)

(511

)

(331

)

(400

)

(222

)

(311

)

(111

)

x=0.40

x=0.20

x=0.15

x=0.05

x=0.10

x=0.30

x=0.25

x=0.35

x=0.45

x=0.50

(a)

32 34 36 38 40 42

(b)

Inte

nsi

ty (

a.u

.)

2 Theta

x=0.00

(222

)

(311

)

x=0.40

x=0.20

x=0.15

x=0.05x=0.10

x=0.30

x=0.25

x=0.35

x=0.45

x=0.50

0 1 2 3 4 58.16

8.17

8.18

8.19

8.20

8.21

8.22

Lat

tice

con

stan

t(A

)

The value of X

0

(c)

Fig. 1 a XRD patterns of LiNixMn2−xO4 (0≤x≤0.5) samples with var-ious molar ratio of Ni/Mn. b The enlarged diagram of Fig. 1a in the 32–42° region. cVariation of the lattice parameters as a function of Ni contentin spinel LiNixMn2−xO4

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typical discharge curve of lithium nickel manganate, the dis-charge capacity is 135 mAh.g−1 with a long and flat discharge

plateau at 4.7 V, and the plateau at 4.0 V is so short that canhardly be observed.

With the decrease of the ratio of Ni/Mn, two phenomenacan be observed (Fig. 4b): firstly, the total discharge capacitiesare decreased gradually with the decrease of Ni content; sec-ondly, the 4.7 V plateau which belongs to the oxidation/reduction of Ni4+/Ni2+ becomes short and the 4.0 V plateauwhich belongs to the oxidation/reduction of Mn4+/Mn3+

becomes long gradually with the decrease of the ratio ofNi/Mn. The length of 4.7 V plateau is almost proportional tothe content of Ni in the samples. The discharge capacity ofLiNixMn2−xO4, of which the value of x is below 0.2 are around108 mAh g−1 except for the sample of x=0.15 exhibiting122 mAh g−1. The other region is around 120 mAh g−1 withthe value of x between 0.2 and 0.45 while the sample of x=0.4exhibits 128 mAh g−1.

Fig. 2 FTIR spectra of LiNixMn2−xO4 samples with various Ni content

Fig. 3 SEM images ofLiNixMn2−xO4 samples:a, b x=0; c, d x=0.2;e, f x=0.4

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Regarding the cyclic stability (Fig. 5), it is found that almostall the samples exhibit good stability at the rate of 0.1 C

(current density of 14.7 mA g−1). Except for three samples atx of 0.45, 0.35, and 0.2, other samples exhibit as good stabilityas LiNi0.5Mn1.5O4, and superior stability to the LiMn2O4, Forthe LiNi0.4Mn1.6O4 sample, its discharge capacity almost stayat the same after 50 cycles. On the other hand, it demonstratesthat the partially substitution ofMnwith Ni which can stabilizethe octahedral spinel sites (binding energy of Ni-O in NiO2 is1,029 kJ mol−1) result in more stable crystal structure, and thusresult in more stable cyclic performance [30].

Conclusion

In conclusion, a series of spinel LiNixMn2−xO4 (0≤x≤0.5)with varies ratio of Ni/Mn have been successfully synthesizedby a co-precipitation method, followed by a solid state reac-tion. And the effect of Ni content on the structure and perfor-mance of the lithium nickel manganate materials has beeninvestigated intensively. It is found that the ratios of Ni/Mnalmost do not affect the basic structure, but the lattice param-eters change slightly with the change of Ni content. The totaldischarge capacity, as well as the length of two dischargeplateaus, is affected greatly by the content of added Ni. Withthe increase of the ratio of Ni/Mn, the capacity of the 4.0 Vplateau is decreased while the capacity of the 4.7 V plateau isincreased, and the total discharge capacity shows growthtrend. It is important that almost all the samples with variousratio of Ni/Mn exhibit good cyclic stability, revealing thestable structure of the materials.

Acknowledgments This work was supported by the National ScienceFoundation of China (NSFC Project Nos. 21076089, 21276098,11132004, U1301245), Guangdong Natural Science Foundation (ProjectNo S2012020011061), Doctoral Fund of Ministry of Education of China(20110172110012), and Doctoral Fund of Department of Education ofGuangdong.

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